Back between 1975 to 1991, they had solid stud head holes. Then, LEGO became concerned that minifig heads might be a choking hazard. So they added small holes to the heads so that if a minifig head got lodged in a child's throat, they could still breathe (if it weren't lodged sideways). So from 1992-2010, minifig heads all had little holes inside the head.

However, LEGO recently decided that it's getting a lot of competition, and has been on a branding kick. That means they're re-evaluating all their elements that DON'T have the LEGO logo on them and attempting to stick it in there. In particular, they wanted to put the LEGO logo on the minifig head. So they re-evaluated the choking hazard on a minifig head, and determined that it wasn't as big of a hazard as they previously thought, and that it would be OK if they did away with the holes, and put the LEGO logo at the bottom of the recessed stud.

Hence, starting in 2011 (not everything, but some things), you'll see some minifig heads that have a hollow stud without holes at the bottom.

So far I have been very fortunate as far as cracked bricks/slopes. I have never had a cracked building piece. However, I have noticed that my torsos tend to crack a lot under the arm hole when I am taking out or putting in an arm for a custom figure. This bothers me, as i already have more legs then torsos. But anyway, this has happenedto me about 3 times, twice with new figs, once with a (really) old fig. Oh well. It still holds the arm, just its loose.